The invention discloses an insoluble dietary fiber which is prepared according to the following steps: based on red date skin and/or red date core, hawthorn and wheat bran as raw materials, cleaning the raw materials; performing high-pressure and high-temperature treatment in an autoclaving pot; and baking and crushing. According to the manufacturing method, the originally wasted red date skin, red date core and inedible wheat bran are turned into the edible and health-care dietary fiber, so that the residue problem of phytic acid in the wheat bran is solved; starch and fat are removed; the excellent taste is achieved by performing microwave baking, puffing and superfine grinding; the insoluble dietary fiber can be added or forced into the food as a food additive; the waste is turned into wealth; and the insoluble dietary fiber has high economic and social benefits.